Fluorinated polymers containing sulfonic acid functional groups, due to their ion conducting properties, have found widespread use in the manufacture of electrolyte membranes for electrochemical devices such as electrolysis cells and fuel cells. Notable examples are for instance proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant.
Fluorinated polymers containing sulfonic acid functional groups have also been known to provide hydrophilic fluorinated surfaces due to the presence of the sulfonic acid groups.
To provide a high proton transport capability to an electrolyte membrane or to efficiently interact with water in a hydrophilic fluorinated surface polymers having a high number of sulfonic acid groups are required, which however are generally provided with reduced mechanical and physical resistance with consequent negative effects on the duration of the articles obtained therefrom.
The use of cross-linking to improve the physical resistance of membranes made from fluorinated polymers containing sulfonic acid functional groups has been previously disclosed. For instance, EP 1238999 A (SOLVAY SOLEXIS SPA) 11 Sep. 2002 and EP 1239000 A (SOLVAY SOLEXIS SPA) 11 Sep. 2002 disclose hydrophilic membranes comprising cross-linkable sulfonic fluorinated polymers comprising: monomeric units deriving from tetrafluoroethylene, fluorinated monomeric units containing sulfonyl groups —SO2F, and from 0.01% to 5% by moles of monomeric units deriving from a bis-olefin of formula (I): R1R2C═CH—(CF2)m—CH═CR5R6 (wherein m=2-10, R1, R2, R5, R6, equal to or different from each other, are H or C1-C5 alkyl groups). The membranes are obtained by cross-linking of the sulfonic fluorinated polymer, the cross-linking involving the backbone of the polymer. The membranes are suitable both for use as ion conducting membranes in electrochemical cells as well as filtration membranes.
Cross-linking of fluorinated polymers involving the sulfonyl fluoride functional group precursor to the sulfonic acid functional group has also been previously described. U.S. Pat. No. 6,733,914 (ION POWER, INC.) 11 May 2004 discloses ion exchange membranes comprising cross-linked polymers having the following structure:
obtained by treatment of the fluorinated polymer comprising —SO2F functional groups with ammonia, followed by hydrolysis of the residual —SO2F functional groups with a strong base and then by heat treatment. The resulting bridging group has however a limiting effect on the water absorption ability of the membrane.
WO 2007/142885 A (E.I. DU PONT DE NEMOURS) 13 Dec. 2007 and WO 2007/142886 A (E.I. DU PONT DE NEMOURS) 13 Dec. 2007 similarly disclose the cross-linking of fluorinated polymers involving the reaction of —SO2F functional groups. In particular these documents disclose the cross-linking of polymers comprising 95% to 5% of —SO2X functional groups (X=halogen) and 5% to 95% of —SO2X functional groups that have reacted with a nucleophilic compound Y with a cross-linkable compound having the potential to form cross-links with the —SO2X functional groups. The cross-linkable compound thus reacts with the residual —SO2X functional groups in the fluorinated polymer. It has to be noted that the cross-linking reaction between the —SO2F (or —SO2X) functional groups and the cross-linking compound may lead to the formation of hydrogen fluoride (or HX) which has to be properly removed from the resulting cross-linked polymer.
Additionally, the resulting membranes comprising the cross-linked fluorinated polymer have to be submitted to hydrolysis and acidification treatment to transform the —SO2X functional groups that have not reacted with the cross-linking compound and the —SO2X functional groups reacted with the nucleophilic compound Y in the ion conducting acid —SO3H form.
From the foregoing it becomes apparent that the need still exist for providing articles, in particular membranes, with improved physical and mechanical resistance by the cross-linking of fluorinated polymers comprising sulfonic acid functional groups without affecting the ion conducting capability and hydrophilicity of the polymer and with a process involving a reduced amount of steps.